1. Field of the Invention
The invention relates to a method for detecting a reflex of the human stapedius muscle in particular for a hearing test, by which the reflex is triggered by means of an acoustic test signal having a frequency of, for example 500 Hz or higher, and with a sound level of at least 65 dB HL; and the impedance prior to and the impedance after the reflex on the tympanic membrane is measured by means of at least one acoustic measuring signal with a frequency of, for example about 220 Hz and with a sound level of clearly below 65 dB HL, said measuring signal lasting longer, if need be, and being compared by forming a difference between the corresponding measuring signals or measuring signal ranges.
Furthermore, the invention relates to a device for detecting a reflex of the human stapedius muscle, with a signal transmitter for emitting acoustic signals or sounds, and with a signal receiver, preferably a microphone for carrying out said method.
2. The Prior Art
In stapedius reflex audiometry of the type concerned herein, a reflex or the human stapedius muscle is used, which can be triggered by an acoustic stimulus, whereby sound stimuli exceeding a level of 80 or 90 dB HL (hearing level) are conventionally offered.
The level threshold above which the reflex of the stapedius muscle can be observed is referred to as the acoustic reflex threshold.
The stapedius muscle is the smallest muscle in the human body. It is located in the middle car and connected with the stapes, one of the auditory ossicles. Tensioning of said muscle changes the mechanical properties of the chain of auditory ossicles and thereby also the acoustic impedance on the tympanic membrane. It can be said in a simplified way that the sound reflection properties of the tympanic membrane change depending on the tensioning of the stapedius muscle, which changes not only the intensity and thus also the amplitude of a reflected sound, but also its phase as compared to the phase of the reflected signal when the stapedius muscle is relaxed.
Said described behavior of the stapedius muscle and the impedance variation resulting therefrom are used in stapedius reflex audiometry for the measurement, for example within the framework of a hearing test or other scientific function tests of a human ear.
The following procedure is conventionally employed in this connection: A continuous measuring sound with a frequency of, for example 226 Hz is emitted, such sound having a relatively lower level not sufficient for triggering the muscle reflex, and a short signal is additionally generated and emitted during the duration of the measuring sound in order to trigger the stapedius reflex. Said signal frequently has a sound level far above 80 dB HL and another frequency, for example the audiogram frequencies 500 Hz, 1 KHz, 2 KHz, and 4 KHz. The test signal also may be wide-banded (e.g. noise) and it may be repeated at short intervals.
As soon as the stapedius reflex has been triggered by the signal, the sound curve received by a signal receiver, for example by a microphone--with the auditory canal closed in most cases--changes due to the changed impedance on the tympanic membrane. Measured is in this connection the change in amplitude of the measuring sound reflected on the tympanic membrane.
Said measuring method exhibits little sensitivity because of the very minor variations of the reflected measuring sound, so that a stapedius reflex is not always safely detectable.
However, a method as described above is known from U.S. Pat. No. 3,949,735, by which the differences between the level of the reaction to the measuring sound prior to and after the reflex are more exactly detected by difference formation. However, a rectifier is employed for this purpose, which makes any later effect of phase information impossible.
Furthermore, the level employed for the test sound in the state of the art frequently reaches the range of the discomfort threshold, which is graded by test persons as unpleasant or insufferable. Therefore, the conventional stapedius reflex audiometry method cannot be applied unlimited with some test persons, in particular not with patients who shortly before suffered a sudden drop in hearing ability.